Communications systems comprising optical fiber links are being installed at a rapidly increasing rate, and it is expected that optical fiber will become the transmission medium of choice in many applications. One of these applications is expected to be undersea cable for voice and data transmission.
Optical fiber is now routinely being produced with losses less than 1 dB/km at wavelengths in the near-infrared, e.g., at 1.3 .mu.m or 1.55 .mu.m, wavelengths that are currently frequently considered for long-haul transmission links. In the laboratory fiber has been produced that has much lower loss still, of the order of 0.2 dB/km, and it can be expected that production fiber will approach such low loss values in the future.
Optical fiber is typically produced by drawing from a glass body, generally referred to as a preform. The preform is produced to have an appropriately radially shaped refractive index profile, and the profile is transferred to the fiber. Typically more than 10 kilometers of fiber can be drawn from a single preform, but the length of cabled fiber typically is at most a few kilometers. Since repeater spacings substantially in excess of the length of single pieces of cable are possible, it is obviously necessary to join one or more cable lengths to complete a fiber span, i.e., a unit of the fiber transmission system. Fiber often also is proof tested prior to cabling, and such tests can result in breakage of fiber at weak points in the fiber, necessitating joining of fiber segments. From the above remarks it is evident that it is necessary to have available methods for joining together pieces of optical fibers.
Prominent among the currently used methods for joining fibers is the fusion method. See, for instance, J. P. Krause et al, Electronics Letters, Vol. 17(21), pp. 812-813, 1981. In fusion splicing, the fiber ends, after aligning the cores to minimize signal loss, are heated by means of a microtorch and caused to fuse. Such splices can be prepared routinely having losses less than 0.1 dB, and relatively high strength.
It has been found that splices of especially high strength, of the order of 4 GPa (about 580 kpsi) or more, can be produced with a H.sub.2 -Cl.sub.2 flame, but splices produced with the aid of a H.sub.2 -O.sub.2 flame are found to typically have considerably lower tensile strength.
Despite the excellent results achieved with the H.sub.2 -Cl.sub.2 flame, the method has considerable disadvantages. Chief among these is the toxicity of chlorine. Due to this toxicity, special precautions are required whenever a chlorine-hydrogen torch is to be used. For the same reason, chlorine transport, storage, and disposal are cumbersome and expensive. These and other considerations militate against use of H.sub.2 -Cl.sub.2 flame fusion on the factory floor and, even more so, against its use on board of submarine-cable-laying ships.
In light of the above, it is clear that a method for fusion-splicing optical fiber that yields high-strength, low-loss splices, that does not involve highly dangerous or toxic substances, and that is relatively inexpensive, would be of great commercial interest. This application discloses such a method.